| The coarsening behavior of coherent gamma Ni-Al and Ni-Ge solid solution precipitates in binary 22.0, 22.2, 22.4, 22.6, 22.8 at. % Ni3Al (gamma') alloys and 22.0, 22.5, 23.0, 23.5 at. Ni 3Ge(gamma') alloys (inverse alloys) was investigated to study the interdependence among kinetics, morphology, spatial correlations and volume fraction, fe. The particle size, shapes and distributions was measured using TEM. First the (gamma + gamma ')/gamma' phase boundaries in the Ni-Al and Ni-Ge phase diagrams were established over the temperature ranges 600--1200°C and 700--1000°C, respectively. The Ni-Al and Ni-Ge alloys were aged between 650 and 800°C and between 600 and 700°C, respectively, for different aging times in order to study the volume fraction dependence of the coarsening behavior.; The gamma precipitates evolve in shape from small, equiaxed spherical to cuboidal to large non-equiaxed plates due to precipitate coalescence as the size increases. The gamma precipitates in inverse alloys become plate-shaped far more readily than gamma' precipitates in normal alloys because of the absence of anti-phase boundaries in the gamma phase. The rate constants for the kinetics of coarsening increase rapidly as the fe increases. This behavior is consistent with the predictions of only one theory, and contrasts dramatically with the coarsening of gamma precipitates in conventional Ni-Al and Ni-Ge alloys, in which the rate constants are either independent of, or decrease anomalously, as fe increases. Coalescence of precipitates is very easy, in stark contrast to the strong resistance to coalescence of gamma' precipitates. We regard this finding as indirect proof of the postulated role of anti-phase relationships during coarsening of gamma' precipitates in normal alloys.; By studying the coarsening behavior as a function of temperature and comparing it with the results of conventional interdiffusion experiments, it is concluded that chemical diffusion is the prevailing mechanism of diffusion during precipitate coarsening. |
